diff --git a/src/pk_core.c b/src/pk_core.c
index ba150ed..e963a48 100644
--- a/src/pk_core.c
+++ b/src/pk_core.c
@@ -1217,6 +1217,14 @@ Var varBitRshift(PKVM* vm, Var v1, Var v2) {
   return VAR_NULL;
 }
 
+Var varBitNot(PKVM* vm, Var v) {
+  int64_t i;
+
+  if (!validateInteger(vm, v, &i, "Unary operand")) return VAR_NULL;
+
+  return VAR_NUM((double)(~i));
+}
+
 bool varGreater(Var v1, Var v2) {
   double d1, d2;
 
diff --git a/src/pk_core.h b/src/pk_core.h
index ff391ce..205ad37 100644
--- a/src/pk_core.h
+++ b/src/pk_core.h
@@ -41,6 +41,7 @@ Var varBitOr(PKVM* vm, Var v1, Var v2);       // Returns v1 | v2.
 Var varBitXor(PKVM* vm, Var v1, Var v2);      // Returns v1 ^ v2.
 Var varBitLshift(PKVM* vm, Var v1, Var v2);   // Returns v1 << v2.
 Var varBitRshift(PKVM* vm, Var v1, Var v2);   // Returns v1 >> v2.
+Var varBitNot(PKVM* vm, Var v);               // Returns ~v.
 
 bool varGreater(Var v1, Var v2); // Returns v1 > v2.
 bool varLesser(Var v1, Var v2);  // Returns v1 < v2.
diff --git a/src/pk_vm.c b/src/pk_vm.c
index e2896c8..1044440 100644
--- a/src/pk_vm.c
+++ b/src/pk_vm.c
@@ -1220,7 +1220,17 @@ static PkResult runFiber(PKVM* vm, Fiber* fiber) {
     }
 
     OPCODE(BIT_NOT):
-      TODO;
+    {
+      // Don't pop yet, we need the reference for gc.
+      Var val = PEEK(-1);
+
+      Var result = varBitNot(vm, val);
+      DROP(); // val
+      PUSH(result);
+
+      CHECK_ERROR();
+      DISPATCH();
+    }
 
     // Do not ever use PUSH(binaryOp(vm, POP(), POP()));
     // Function parameters are not evaluated in a defined order in C.
diff --git a/tests/lang/basics.pk b/tests/lang/basics.pk
index c197c4f..f14b3c8 100644
--- a/tests/lang/basics.pk
+++ b/tests/lang/basics.pk
@@ -73,6 +73,11 @@ assert(8 >> 1 == 0x4)
 assert(8 >> 2 == 0b10)
 assert(0xa >> 1 == 5)
 
+assert(~8 == -9)
+assert(~1 == -2)
+assert(~0b0 == -1)
+assert(~0x1 == -2)
+
 x = 42  ; assert((x&=51) == 34)
 x = 123 ; assert((x&=324) == 64)
 for i in 0..1000